Natal homing

Natal homing, or natal philopatry, is the homing process by which some adult animals that have migrated away from their juvenile habitats return to their birthplace to reproduce. This process is primarily used by aquatic animals such as sea turtles and salmon, although some migratory birds and mammals also practice similar reproductive behaviors. Scientists believe that the main cues used by the animals are geomagnetic imprinting and olfactory cues. The benefits of returning to the precise location of an animal's birth may be largely associated with its safety and suitability as a breeding ground. When seabirds like the Atlantic puffin return to their natal breeding colony, which are mostly on islands, they are assured of a suitable climate and a sufficient lack of land-based predators.

Sea turtles born in any one area differ genetically from turtles born in other areas. The newly hatched young head out to sea and soon find suitable feeding grounds, and it has been shown that it is to these feeding areas that they return rather than to the actual beach on which they started life. Salmon start their lives in freshwater streams and eventually travel down-river and are washed out to sea. Their ability to travel back, several years later, to the river system in which they were spawned is thought to be linked to olfactory cues, the "taste" of the water. Atlantic bluefin tuna spawn on both the east and west shores of the Atlantic Ocean but intermingle as they feed in mid-ocean. Juvenile tuna that have been tagged have clearly shown that they almost invariably return to the side of the Atlantic on which they were spawned.

Various theories have been put forward as to how the animals find their way home. The geomagnetic imprinting hypothesis holds that they are imprinted with the unique magnetic field that exists in their natal area. This is a plausible theory but has not been proven to occur. Pacific salmon are known to be imprinted on the water chemistry of their home river, a fact that has been confirmed experimentally. They may use geomagnetic information to get close to the coast and then pick up the olfactory cues. Some animals may make navigational errors and end up in the wrong location. If they successfully breed in these new sites, the animal will have widened its breeding base which may ultimately increase the species' chances of survival. Other, unknown means of navigation may be involved, and further research is needed.

There are several different kinds of marine animals that demonstrate natal homing. The most commonly known is the sea turtle. Loggerhead sea turtles are thought to show two different types of homing. The first of which comes in the early stages of life. When first heading out to sea, the animals are carried out by tides and currents with little swimming involved. Recent studies now show that the animals demonstrate homing to feeding grounds near their natal birthplace.

Turtles of a specific natal beach show differences in their mitochondrial DNA haplotypes that distinguish them from turtles of other nesting areas. Many turtles from the same beaches show up at the same feeding areas. Once reaching sexual maturity in the Atlantic Oceans, the female Loggerhead makes the long trip back to her natal beach to lay her eggs. The Loggerhead sea turtle in the North Atlantic cover more than 9,000 miles round trip to lay eggs on the North American shore.

The migration of North Pacific Salmon from the ocean to their freshwater spawning habitat is one of the most extreme migrations in the animal kingdom. The life cycle of a salmon begins in a freshwater stream or river that dumps into the ocean. After spending four or five years in the ocean and reaching sexual maturity, many salmon return to the same streams they were born in to spawn. There are several hypotheses on how salmon are able to do this.

One hypothesis is that they use both chemical and geomagnetic cues that allow them to return to their birthplace. The Earth's magnetic field may help the fish navigate the ocean to find the spawning region. From there, the animal locates where the river dumps into the sea with the chemical cues unique to the fish's natal stream.

Other hypotheses rely on the fact that salmon have an extremely strong sense of smell. One hypothesis states that salmon retain an imprint of the odor of their natal stream as they are migrating downstream. Using this memory of the odor, they are able to return to the same stream years later. Another smell-related hypothesis states that the young salmon release a pheromone as they migrate downstream, and are able to return the same stream years later by smelling the pheromone they released.